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What exactly is a "coordinated" turn?
I am wondering what turn coordination really means and what makes a turn uncoordinated?
I know that when the turn is coordinated, there is no slip and skid, an aircraft is flying a perfect circle etc. and that in one case there is too much rudder input or too little bank etc. etc. But I am interested about forces during the turn. Can someone tell me which force during an uncoordinated turn is too big or too small and literally what makes a turn uncoordinated in terms of forces?

Can someone tell me which force during an uncoordinated turn is too
  big or too small and literally what makes a turn uncoordinated in
  terms of forces?

The slip-skid ball (inclinometer ball) will be off-center whenever the component of the net aerodynamic force that we see when looking at the airplane in a head-on view is tilted "sideways" in the aircraft's reference frame, rather than acting "straight up" perpendicular to the wingspan.
That's really all there is to it.
To a first approximation, this only can happen when the fuselage is flying sideways through the air and generating an aerodynamic sideforce, although there are some additional nuances to be considered (e.g. the helicopter case and the twin engine aircraft w/ one engine out case.)
The direction and magnitude of the "load" vector is irrelevant-- the perceived "load" is nothing more than the mirror image of the real aerodynamic force vector acting on the aircraft.
The direction and magnitude of the "weight" or "gravity" vector is irrelevant-- gravity exerts an equal force per unit mass (acceleration) on every molecule of the aircraft and contents, including the slip-skid ball and its surrounding glass tube, and the pilot and the seat he or she is sitting in.  Gravity doesn't pull the pilot to one side of the aircraft, or pull the slip-skid ball to one side of its tube.
The magnitude of the "centrifugal force" vector is irrelevant-- "centrifugal force" is not a real force and is only a valid concept from the point of view of the (accelerated) reference frame of the aircraft itself.  The "centrifugal force" vector is simply the mirror image of the centripetal force vector, which is the component of the real aerodynamic force that acts in the centripetal direction.
The magnitude of the lift vector is irrelevant-- moving the stick aggressively forward or aft will dramatically change the magnitude of the lift vector and net aerodynamic force and G-loading (which is just another word for net aerodynamic force, or for the lift vector), and also will dramatically change the turn rate, but has little effect on the slip-skid ball.
It's a basically a mistake or at least an un-useful concept to think uncoordinated flight (slip or skid) is CAUSED by flying at the wrong turn rate for the bank angle.  Rather, allowing (or forcing) the aircraft to fly sidewise through the air CAUSES the yaw string to be off-center and ALSO creates an aerodynamic sideforce which CAUSES the turn rate to be wrong for the bank angle and ALSO causes the inclinometer ball to deflect off-center.  Hence rudder usage is key to avoiding (or causing) slips or skids.  WHY a given rudder input would be required to prevent slip or skid, is a complicated aerodynamic question.  In some cases no rudder input is required at all.
Yet in truth the situation is a little more nuanced -- a helicopter tail rotor can create a sideforce that will offset the inclinometer ball even the yaw string is centered (no slip or skid) and the fuselage is not flying sideways through the air, and so for that matter can a deflected rudder when it is opposing some other yaw torque such as that due to having only one engine running in a twin-engine aircraft.  So from another perspective it can be both true and useful to realize that the inclinometer ball will be off-center if the turn rate is "mismatched" to the bank angle and G-loading-- even if the yaw string is centered.  Whether the aircraft is "coordinated" or "uncoordinated" in such a case is a matter of semantics.  Certainly an aerodynamicist would not say it is "slipping" or "skidding", because there is no sideways airflow over the aircraft.
Even in these nuanced cases, the basic rule applies-- the slip-skid ball (inclinometer ball) will be off-center whenever the component of the net aerodynamic force that we see when looking at the airplane in a head-on view is tilted "sideways" in the aircraft's reference frame, rather than acting "straight up" perpendicular to the wingspan.  In the case of the helicopter tail rotor, the sideforce is coming from the tail rotor itself rather than from airflow striking the side of the fuselage.  In the case of the aircraft with strongly deflected rudder (to compensate for a failed engine in a twin-engine aircraft, or for any other reason) the sideforce is coming from the rudder itself rather than from the airflow striking the side of the fuselage.
In most cases-- a twin-engine aircraft with one failed engine and strongly deflected rudder being a notable exception-- the sideforce generated directly by an aircraft's rudder can be ignored.  Note that the sideforce from the rudder itself acts to move the slip-skid ball to the left when the rudder is deflected to the left, because the rudder is pushing air toward the left and creating an aerodynamic sideforce toward the right.  In most cases when we move the rudder to the left we actually see the slip-skid ball move toward the right, because we are exposing the right side of the fuselage to the airflow, pushing air toward the right and creating a leftwards aerodynamic sideforce that dominates over the small rightwards aerodynamic sideforce created by the rudder itself.
One thing is certain-- if the plane is moving sideways through the air, and your analysis of the forces at play isn't considering the real, tangible aerodynamic sideforce generated by the resulting sideways airflow impacting against the side of the fuselage, then you aren't seeing the whole picture.  We find this mistake in many pilot ground school materials, as well as in FAA "written" test questions and in study guides for FAA "written" tests.
IF you are defining uncoordinated flight as the inclinometer ball being off center, THEN you can say the cause is that the turn rate, and the resulting apparent "centrifugal force" (which is not a real force), are mismatched to the bank angle and G-load.  To a FIRST APPROXIMATION, this means that aerodynamic sideforce generated by the fuselage is not zero, i.e. the fuselage is being allowed to fly sideways through the air, but as noted above this is not ALWAYS the case.
IF you are defining a slip or skid as the yaw string being off center, THEN you can say that the cause is that the rudder is not being applied as needed to bring the net aerodynamic yaw torque to zero when the nose of the aircraft is pointing directly into the oncoming airflow (relative wind).  Thus causing the aircraft to adopt a different orientation to the oncoming airflow (relative wind), where the net yaw torque is zero.
WHY some rudder deflection is often needed to prevent the plane from flying slightly sideways through the air in turning flight, is a complicated matter best addressed in another question.  However we can start by noting that the outboard wingtip must move faster through the air, and thus tends to experience more drag, than the inboard wingtip.  We can also note that the fuselage can't curve itself like a banana to accomodate the curving relative wind (airflow) in the turn, so when the vertical fin "weathervanes" into alignment with the direction of the airflow at the rear of the aircraft, more forward locations such as the CG and the nose tend to experience a sideways component in the airflow.

Thank you, great answer, that is clearing a lot of misunderstandings. So to sum up, uncoordinated turn might be „properly” defined either by the yaw movement or by the inclinometer ball displacement? And it is not impossible to properly explain uncoordinated turn via imbalance of four forces - lift, weight, centrifugal, centripetal.

Okey, thanks for your engagement!

@Konrad-- went ahead and included some of this in a first go at a revision but you may see another upgrade in the future.

Future edit-- could change "The direction and magnitude of the "load" vector is irrelevant-- the perceived "load" is nothing more than the mirror image of the real aerodynamic force vector acting on the aircraft." to "..... generated by the aircraft."

Future edit-- add links to ASE questions/ answers or outside sources addressing the twin-engine situation in more detail.  Likewise explaining the glider yaw string in more detail.

Future edit-- change "one engine out" to "one failed engine".

Future edit-- note that GIVEN certain constraints such constant G-load, no acceleration in the vertical direction, using the aircraft as our reference frame (not a valid inertial reference frame), it is valid to say that the inclinometer ball will be off-center whenever the turn rate, and therefore the perceived centrifugal force, is mismatched to the bank angle., thus creating an imbalance between centrifugal force and gravity. However this explanation doesn't explain WHY this situation might arise.

Future edit ctd-- The explanation given in the first few paragraphs of this answer is the more general one-- it applies even during aerobatic maneuvers--  and also explains ALL the actual forces involved.

Future edit-- answer could be re-organized to first start with the basic assumption that the ONLY significant source of aerodynamic sideforce is the fuselage moving sideways through the air, and then only later take up the rudder/ one failed engine effect and the helicopter tail rotor effect as an extra nuance.

Future edit-- note that in theory, any time we are holding some rudder deflection to keep the yaw string centered, we are creating a sideforce that has some small effect on the slip-skid ball, just as in the case of the twin-engine airplane with one failed engine.  The effect is usually trivial, but it does exist.  So in theory if we are holding lots of rudder deflection and we have no yaw string, we should let the slip-skid ball stay slightly off center. I.e. we should. apply slightly less rudder than would be needed to fully center the slip-skid ball.

Future edit ctd-- In such a case, if we are trying to fly in a straight line (constant heading), this means we'll be maintaining a slight bank angle. If we are holding lots of rudder and flying in a straight line with the ball centered (and therefore the wings level), the yaw string cannot be completely centered, The fuselage must be flying slightly sideways to offset the sideforce from the rudder itself. Could pertain to maximizing efficiency in a prop plane in a climb at low airspeed and high power setting. Might be undetectable in actual practice.

Future edit- point out that in the specific case of unaccelerated constant-heading flight, the slip-skid ball functions as a bank angle gauge.

@quiet flyer, good grief - it's been a year and a half... edit your answer already!!!

It's important not to overthink it.  I'll keep it simple.  A coordinated turn means you are keeping the tail lined up with the nose in the airstream.  If you are uncoordinated, you are flying sideways in the airstream to some degree or another; the side of the fuselage is being presented to the airflow.
If you learn to fly in gliders, it's obvious because you have a yaw string on the canopy to tell you that the tail is lined up behind the nose in the airstream, since the string itself allows a direct visualization of the airstream.  In power planes with a propeller up front, you are forced to use an inclinometer to tell you the same thing, because it detects the lateral acceleration relative to the vertical axis that is present if the tail is not lined up behind the nose.
Practically speaking, whether you are banked in a turn or in level flight, it really doesn't matter.  The fuselage is lined up in the airstream or it's not.  The ball is centered, or it's not.  Just use your feet to keep it lined up as required using the skid ball.

Twin-engine airplane, one engine failed, rudder applied as needed to center the yaw string.  Is the ball centered?  Or, rudder applied as needed to center the ball.  Is the yaw string centered?

I had to chuckle at the glider example. An R-22 has the same yaw-string. I did not think it would work due to rotor-wash. But it does. It is more for efficient flight than coordinated flight since it applies to straight & level, unaccelerated flight as well. The helicopter can fly perfectly well sideways. After a while, you tend to ignore it when flying VFR. With the doors off to save weight, you can hear and feel whether the flight is streamlined. At a certain speed, the aircraft nose and tail will properly align themselves without rudder input due to weathervaning of the vertical stabilizer.

@quietflyer if you don't lower the wing into the dead engine, the yaw string will be offset, but the skid ball will be centered, because the net thrust line is offset by the sideways thrust of the rudder.  Lowering the wing into the dead engine creates a sideslip component that cancels out the lateral thrust being generated by the rudder.  You will be flying straight through the air, the yaw string will be straight, and the ball will be offset toward the lowered wing. Ball is offset in coordinated flight because the offset location of the inclinometer is the true "down" location in this case.

Yeah once you're in translational lift and speed up, the thrust wash of the rotor is angled back quite steeply so that a string on front of the canopy is not in the rotor wash.  Some pilots with light twins put yaw strings one since the only place they don't work is on tractor engine singles.

If I owned a light twin (in some other lifetime perhaps) it would definitely have a yaw string. I have owned a lightsport pusher and my first modification was to add a yaw string.   One notable thing about a twin-engine aircraft (of conventional configuration with one engine on each wing) is that not only does a yaw string work well because it is out of the propwash, but it is actually a  far superior instrument to the slip-skid ball as a guide to how much rudder to apply to best streamline the fuselage to the airflow if one engine has failed.  Reasons for this are touched on in my answer.

And also are addressed in your comment.

@John K, I thought the neumonic was "raise the dead"?

@Micheal Hall Oops nice catch. It should say "live".

Yeah single engine in a twin is very strange. You are banked, but are flying straight while tilted sideways, prevented from turning into the bank and slipping into the low wing by the rudder's side force summed with the engine's offset thrust line. When I was flying CRJs, banking into the live engine wasn't done, I think because less bank than the 5 deg you use in a piston twin was theoretically required in a jet and it wasn't worth it. At least I think what was the reason we just kept wings level during an engine cut; nobody was ever able to confirm it.

If you are asking about the forces involved in an uncoordinated turn, Thrust, Drag, Lift, and Load (weight) still apply. But, let us define Lift as acting perpendicular to to the wings in a direction opposite Load. While Load is acting perpendicular to the wings in the direction of gravity plus the aircraft’s momentum. When you are banked, or you are in a turn, Load is acting still perpendicular to the wings. Except, now the direction is not straight down. It is split between the downward component and the sideways or lateral component. If the Lift components are not kept in balance with the Load component, you will experience uncoordinated flight. In a slip, you, the pilot, will feel the lateral forces pull your body toward the center of the turn. In a skid, you will feel the lateral forces pulling you to the outside of the turn. In coordinated flight, the pilot feels the forces increase in a direction that, to them, feels straight down into their seat.

To oversimplify it, think of it like turning your car. A coordinated turn will see your back wheels always following your front wheels (albeit a little more to the inside of the turn). If you overcome friction to break traction between your front tires and the ground, you will perform a slip called snow plowing. If you loose traction in your back tires, you will perform a skid called fishtailing. You might not feel too much of the lateral forces in a slip. It may feel more like the momentum of the car. You will feel more of the lateral forces toward the outside of the turn in a skid. Either way, your nose is not following your tail. It is uncoordinated.
The same is true in an airplane. And just like in a car, uncoordinated flight in an airplane can be advantageous depending on your circumstances and desired outcome. The problem with an airplane is that you need a certain velocity (direction and intensity) of airspeed over each wing to continue flight. If you fishtail (swing your tail) to the outside of the turn, you slow down the wing on the inside of the turn and speed up the wing on the outside of the turn. With enough of a change in velocity, you can stall the inside wing while simultaneously creating an excess of Lift in the outside wing. This will lift the outside wing and drop the inside wing creating a dangerous spin scenario.
Another concept to consider is the unintended and undesirable yaw created by the downward pointing aileron creating more Lift than the upward pointing aileron. This creates more drag on the side of the downward pointing aileron. This adverse yaw is also a contributing factor in making a turn uncoordinated. The nose will turn in a direction opposite that of the bank and intended turn in a type of snowplowing. This will cause excess drag due to the airplane’s aerodynamics no longer being streamlined. This increase in total Drag will cause the aircraft to Slow down or not fly as fast as it should in a given flight regime.
The Slip/Skid Indicator on your Turn Coordinator acts like a calibrated pendulum. It will move in the direction that the forces acting on the plane will move any object in the plane (including the pilot). In other words, it will move to the outside of the turn in a skid, and to the inside of the turn in a slip. Applying rudder pressure in the same direction (or easing/reducing rudder pressure in the opposite direction) will correct the slip or skid.

Am I correct to conclude that a motor-cyle is always involved in coordinated turns when the tyres are maintaining traction except when in the process of falling off :-). (Dynamic movements such as power slides, jumps and wheelies appear to be exceptions :-) ).

@quietflyer - “The importance of the aerodynamic force created by the airflow striking the side of the fuselage during a slip or skid” was covered in the statement “ This will cause excess drag due to the airplane’s aerodynamics no longer being streamlined”. I figured this was adequate explanation for anyone familiar with a forward slip. But, your comment spells it out as well.

@RussellMcMahon A motorcycle behaves (essentially) the same way in a turn that a car does. If the front wheel looses grip, you're pushing/under steering/slipping. If the back wheel looses grip, you're loose/over steering/skidding. The motorcycle can bank like an aircraft to help prevent the over/under steer, but, unlike in a car, too much over/under steer will leave you under the bike or shooting off like a rocket, and in either case, less protected than your 4-wheeled bretheren.

@Freeman - on road, this is not true for any of the 25 or so motorcycles that I've owned. Except, when a sidecar is fitted. Agh !!!!

@ Russell McMahon, no, actually it is opposite. If you are in a perfectly coordinated turn on a road then you don't even need traction at all! It is horizontal component of normal force (aka centripetal) that causes direction change to turn so you don't need to rely on traction force to keep you in the turn. That only happens if you are on inclined ramp at just the right speed for that angle & radius of curvature. At any other point in time you will be uncoordinated, and it is traction that allows you to make a turn and keeps you in the turn within small +/- tolerance...until it's not enough.

A simple way to describe it is coordinated flight is one in which the fuselage of the aircraft is aligned with, or is tangent to, the direction of flight.  This may or may not result in a centered ball on a quality-of-turn indicator, but will always result in a centered yaw string as the fuselage is aligned to the relative wind.  Lack of coordination in a turn follows when the the fuselage is not aligned with the relative wind.  Lack of coordination results when the horizontal component of lift from the wings is not balanced with the centrifugal force imposed on the aircraft from the turn and a net side load is imposed.  It can also be a byproduct of an asymmetric drag loading on the airframe eg adverse yaw caused by aileron deflection.  This causes the aircraft to slip or skid in the turn.  This can be corrected by with the use of rudder or anti-torque input as in the case of a helicopter.  Large deflections of rudder to counter asymmetrical drag, such as the case of an engine failure in a multiengine aircraft also require a shallow bank angles away from the direction of slip to maintain coordinated flight.  This will result in an slightly uncentered ball, but an aligned yaw string.

In a coordinated turn, the weight of your body is always downwards on your seat. That makes it comfortable for the pilot as well as passengers.
You might have noticed while travelling in an aircraft that, say your plane is taking a left turn, but you don't feel the plane taking any turn, only to realize it when you look out the window (the landscape is tilted way too much!). That's because the left rolling action (which would have made your weight shift to the left) is compensated by the left rudder (which makes the plane have an imbalanced counterclockwise torque at the tail, makes the plane turn left, and the centrifugal force makes your weight shift to the right).
In cars having stiffer suspensions, taking a turn even at slower speeds makes passengers experience centrifugal forces. This is due to the restricted rolling movement of a car's body.

It isn't all that difficult if you understand the three different axis, and the corresponding rotation about each axis.  (roll, pitch, and yaw)
In your question you described a coordinated turn.  (no slip or skid)  An uncoordinated turn is simply when there IS slip or skid.  This occurs where there is either not enough, or too much yaw.  This happens when you apply either not enough, or too much rudder input.

Yes, I understand the necessary control inputs during slip or skid, but I don’t know which forces are unbalanced during for example skid. That is my question

The same force that causes Yaw.  Either rudder, or adverse yaw from the ailerons.  Is that not clear in my answer?

Maybe I'm not writing it clearly or I'm not understanding it properly and it is impossible to explain it in that way or sth. During turn there are 4 forces - lift, weight, centripetal and centrifugal force. Is it possible to explain uncoordinated turn by some of this four forces imbalance?

OK, I follow you a bit better now.  Well, it isn't lift or weight, and if you know enough about centripetal and centrifugal force to list them here I would think you would have no problem discerning their effect.  I guess I am just not understanding your confusion.  Are you looking for engineering equations, or just trying to grasp the big picture of why the ball isn't centered?  NVM, it looks like you are getting a better answer...

Not sure what you mean by that.  Torque is just a force acting on a moment arm.  Was that comment directed at me?

I'm searching some kind of rule (if such rule exist) ,which determines what cause skid and what cause slip, but I'm not talking about explanation like more bank or more rudder. I'm searching for explanation that involves forces acting on the airplane and in which that forces cause unbalanced turn


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Does thrust output in jet airplane remains constant when altitude increases?
I’m a noob so sorry for my ignorance. The question is: do we need (in jet airplane) to increase the throttle to maintain constant thrust output if we want to fly higher (not considering climb phase), I mean whether on 2000 ft and 20000 ft altitude throttle is in the same position or we need to push it forward if we fly at higher altitudes to maintain constant power (as in piston airplanes).

It depends, as so often.
With the thrust available at low altitude, the indicated flight speed will be much higher than at high altitude. There, the reduced density and thrust will only allow flight close to maximum thrust and over a small speed range around the minimum drag polar point. In practical terms, at low altitude the airliner will not be able to fly level at the maximum thrust setting because it will exceed its Mach limit. But since the speed of sound is higher in warmer air, it will still fly significantly faster than at high altitude when it flies at the same Mach number at both altitudes.
If, on the other hand, you compare states at the same indicated air speed, the required thrust at low altitude will be a bit smaller due to the higher Reynolds number in warmer air. On the other hand, fuel flow will be a bit higher since the engine is not running at its optimum design point and the higher ambient temperature reduces its thermal efficiency.
While thrust will go down with air density (or dynamic pressure, for the same Mach number at low and high altitude) in the first case, in the second case it will increase slightly.

No. Power - Thrust or torque - on an air breathing combustion engine varies almost exactly in proportion with air density, so you will need every bit of power in a climb - full power all the way, engine temp limits permitting.  A good rule of thumb is that every 15,000' the density halves and so does your power (unless it is a piston engine with forced induction, but power will still drop off).  By 51,000', the highest certifiable altitude for (subsonic) civil aircraft, the density is 11%.  This means your engine produces 11% of its sea level thrust.  Granted the air resistance is 11% as well, so you need less power.  Jets will burn all their fuel in minutes at low altitude, but can cruise for long periods at altitude, preferably at 40,000' and higher.  The most powerful aircraft can take off with a very aggressive climb rate, even vertical for fighters, but this will drop off to a shallow crawl at higher altitudes when density and power drops.  Forced induction is the answer for piston engines.  For jets and turbo props, they are designed with high compression ratios.
As for wondering about changing the throttle setting - it is common for turboprop engines to get "flat rated", which is like a governor.  It is automatic, but in a sense, this could be thought of as gradually increasing the throttle setting.  Flat rating an engine limits their power at lower altitudes where it is not needed as much, and it also keeps the engine operating at lower limits throughout its life, which allows for longer overhaul intervals and cheaper overhauls.  For example, you could have a 1200 hp engine that gets flat rated to 800 hp and it will simply operate at 800 hp in takeoff and climb till, say, 10,000' and then the power output will start to drop off after that.

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What is RNAV transition and what is the difference between RNAV transition and RNAV STAR
Can someone explain what is RNAV transition and what is the difference between RNAV transition and RNAV STAR. Because sometimes both types of procedures are published for an airport and I don't know what is the diference between those both types.

https://www.faa.gov/air_traffic/publications/atpubs/pcg_html/glossary-s.html#%24SEGMENTS%20OF%20A%20SID/STAR

@randomhead Can you turn that into an answer?

An RNAV STAR will have one or more transitions that you have to file over to begin flying the STAR. The CHPPR ONE STAR to KATL has four transitions: BBABE, LEMKE, MTHEW, and RUTTH. When flight planning into KATL from the northwest (provided your aircraft qualifies per the notes) you would choose one of the transition fixes to begin flying the STAR.
Please also note that two of the four transitions are only for use by departures off of specific airports. BBABE and LEMKE are only used by departures off of KHSV and KCHA (respectively).

The first fix of the arrival portion is often a (null) transition itself.  Ditto for the last fix on a departure.

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Why is a Mode S transponder required to be installed and working for TCAS II to be operational?
I am wondering why a Mode S transponder is required to be installed and working for TCAS II to be operational? I read such phrase in Eurocontrol ACAS II guide and I don't understand why mode A/C is not sufficient for TCAS II to be working.

Short answer
That's a good question since a TCAS II could send and receive coordination messages using its own transceiver and antennas. Why does it use another mean of communication in the first place?
This is to centralize all transmissions done on 1090 MHz, so that they don't overlap with SSR and ADS-B transmissions. The transponder is responsible for managng transmissions on the downlink frequency 1090 MHz.

Frequency management
While a TCAS II is able to listen to other aircraft on 1090 MHz, it doesn't transmit on this frequency. 1090 MHz transmission is completely delegated to the transponder, because:

TCAS messages must be correctly sequenced with SSR mode S replies on 1090 MHz and ADS-B messages, in order to prevent overlapping. It makes sense to use the transponder to decide when to send messages originating from these different sources.
TCAS II coordinated resolution process involves direct aircraft-to-aircraft communication. A mode S transponder is granted a unique address, and this address is used to select the target aircraft.

So

1030 MHz

is listened to by SSR transponder.
is used for transmission by ACAS II to trigger transponder replies from aircraft in the vicinity
is used for transmission by ACAS II to broadcast their initial resolution message.

1090 MHz

is listened by ACAS II
is used by ACAS II to reply to another ACAS II.
the actual transmission is delegated to transponder. In order to allow resolution coordination, this transponder must be mode S.

Coordination
ACAS II protocol, which current implementation is TCAS II 7.1, is used to detect aircraft and exchange resolution data when working with another aircraft equipped with another ACAS II. This exchange is the last phase of ACAS II 3-phase sequence: Detection, Surveillance and Coordination.
The coordination phase includes for ACAS-ACAS resolution the exchange of  vertical resolution advisory complement (vertical RAC or VRC), which can tell the intruder, for instance, "do not pass below", because the aircraft has itself prepared/issued a resolution advisory consisting in descending.

If no VRC has been received the RA (resolution adivsory) is based on the encounter geometry alone. This is what happens when the resolution is initiated by the first aircraft.
If a VRC has been received the RA is determined taking into account the restriction received. In case of conflict, because the two aircraft initiated the resolution at the same time, the one with the lowest ICAO address has the priority.
Once the RA has been determined, each aircraft continuously broadcast messages with the VRC restriction, so that the other aicraft can be sure the two RAs are still complementary.

Such VRC are send using messages carried by a transponder mode S 112-bit frame, and therefore a transponder mode S on both sides is required for coordinated resolution. ACAS II capability when the other aircraft is equipped with different systems:

Source
Some details below, and for how mode S transponder is used as a modem by ADS-B and ACAS, see also What's the relation between ADS-B and MODE S.
Mode S messages used
DF and UF are mode S message formats used respectively for downlink and uplink communication, described in mode S protocol.
To detect other aircraft, ACAS II listens to:

DF11 messages  (56 bits) sent by transponders mode S when interrogated by SSR and by transponders to acquire other aircraft.
DF17 messages (112 bits) sent by ADS-B out.

To initiate a session with a detected aircraft, ACAS II sends DF0 messages (56 bits) at short interval until the other aircraft answers, and other DF0 messages are sent as soon as the other aircraft enters the surveillance perimeter.
If the other aircraft enters the resolution perimeter, it is considered a threat, a coordinated resolution sequence is initiated:

Message UF16 starts the sequence. It contains a MU field.
Messages DF0 (56 bits) and/or DF16 (112 bits), are continuously exchanged. DF16 messages contain a  (MV) field.

MU and MV fields have a length of 56 bits and carry:

Information about aircraft ACAS resolution capability: No capability, vertical resolution, vertical and horizontal (horizontal resolution is for the future, there are no current implementations).
Whether the resolution involve 2 or more aircraft.
What the other aircraft can do vertically or horizontal when selecting the resolution action: Passing above or below may be prevented first, and if this is not possible reallowed to continue negociating the resolution. Another information is used for left/right turns. These data are known as vertical (VRC) and horizontal (HRC) resolution adivsory complement (RAC) and the cancellation as CVC an CHC.

MU field:

Source
MU and MV fields with a length of 56 bits are sent using longer 112-bit transponder messages.
ACAS II has a degraded mode based on transponder mode A/C replies, but in that case no RA coordination is possible. ACAS II stops working if the transponder fails.


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What is the difference between leasing and chartering?
As written above, what is the difference between leasing and chartering? Is it the same thing and if not what are the differences?

Leasing is just a long term rental of the chattel, the airplane itself, where the lessee entity does whatever it wants with the airplane during the period of lease. So you lease an airplane from a leasing company like GE Capital, and do whatever you want with it until the lease is up, then return it.
A charter is a more restrictive and objective-focused contract: it's an agreement to use aircraft for a specific purpose defined in the charter contract.
So if I want an airplane for a year to be at my beck and call to do whatever I wish, I lease it for a year.
If I want an airplane for a specific purpose, like transporting my football team around between cities X, Y  and Z for a month, with no other uses allowed unless specified in the contract, I charter it.
Usually, the charter contract is "turn key" and will include crew and support services, whereas with a lease, the user is only getting the airplane and has to provide all that themselves.
You can also have all-inclusive leases, called "wet leases" that include crews and maintenance, possibly even fuel, built into the lease rate, but you still have freedom to use the plane however you wish, as opposed to a charter where the use of the aircraft is specified in the charter.
An airline might wet lease an airplane when they need a short term replacements for grounded aircraft, especially if the type is different from their fleet.

There is also a difference between a "dry lease" which supplies only the aircraft as you describe, and a "wet lease" which also supplies flight crew and possibly even fuel at a fixed rate. Airlines will sometimes take wet leases from other airlines, while GE Capital would more likely offer dry leases only.

Thanks.  That facet slipped my mind and I've added some text to cover it.


text:
Impact of centre of gravity position on stall recovery
Can somebody explain what is the impact of CoG position on stall recovery. I heard that stall speed is higher with forward CoG comapared to aft CoG, but does it have influence on stall recovery? Is it more or less difficult with some CoG position?

It sure does, and that is the reason we don't abuse aft CG limits.
An excessively aft CG shortens the torque arm of the tail pitch down forces and lengthens the torque arm of pitch up forces ahead of CG, including more of the wing.
As a full stall develops, the nose of the fuselage will also generate lift in addition to its area drag as the plane sinks, making it harder to "bring the nose down" and unstall the wing by reducing its AOA to the relative wind.
By design, a larger tail adds safety if one is foolish enough to test aft CG limits. A speed brake or parachute is another line of defense.
The effect of forward set CG on stall speed may arise from the need to generate a bit more lift with the wing to counteract increased tail downforce but remember most tail "downforce" is used to set wing AOA, only a little more is needed for forward CG.
Forward set CG will also make it harder to lift the nose in slow flight, which may actually drop the nose before it stalls if elevator is small or travel is limited and the horizontal stabilizer is large.  The B-52 was designed in this manner for obvious reasons.
Each plane is different, and even a docile trainer like the Cessna 152 can be dangerous if aft CG limit is exceeded.


text:
What is the relation between manifold pressure, RPM, and power, in a constant speed prop?
I know that there are similar topics here, one is mine even, but still I have few questions about constant speed propeller. My questions:

Why if we decrease RPM and maintain throttle lever setting the power output decrease?
Sounds like very basic knowledge but I heard a lot of versions and I want to reassure. Is tachometer on constant speed prop reading engine rpm or prop rpm?
If we move prop lever we are changing blade pitch and then rpm adjust to it or we are changing engine rpm and blade angle adjust to new rpm

I'm not sure your presumption in #1 is entirely correct.  When prop RPM is decreased the torque load on the engine goes up.  You can see this on the torque gauge of a turboprop.  I believe manifold pressure of a recip would also increase.  I'm not sure how this mathematically affects the total power output, but the engine is definitely working harder.

Why not take a ground lesson with an instructor to get this sorted out?  Power output is one thing (rate of fuel burn), thrust output is another. I would study the power output vs rpm curve (don't forget engine friction).  The peak efficiency rpm is what you want in cruise (engine efficiency x prop efficiency) along with optimal prop pitch.  For take off, we get a little more $thrust$ by using higher rpm and less prop pitch, it's just not as efficient.

Surely I will, but I haven’t flow any aircraft with variable pitch prop yet so all I know is from theory.

1. Why if we decrease RPM and maintain throttle lever setting the power output decrease?
Power is energy per unit of time, and since there is fewer revolutions, there is fewer power strokes to release that energy. There is less contention for the air from intake manifold so the manifold pressure will increase a bit and each power stroke will deliver a bit more energy, but not nearly enough to compensate the overall lower number of power strokes.
For this reason, manifold power can only be related to power for constant RPM, and it's still complicated by other things, especially the density altitude. Fuel flow is a better indicator of power as long as you keep the engine properly leaned.
2. Sounds like very basic knowledge but I heard a lot of versions and I want to reassure. Is tachometer on constant speed prop reading engine rpm or prop rpm?
Usually propeller, but may depend on the aircraft. But it does not really matter, because the gear ratio is always fixed and because the pilot does not have any other use for the value than comparing it to some nominal values in the operating handbook, which are aircraft-specific anyway.
In one aircraft the POH might say that maximum RPM (more can't be selected anyway) is 2500 and most efficient for cruise is 2100, but on another aircraft it might say that 2750 is for take off, but for 2 minutes only, 2600 is maximum for continuous operation and 2300 is most efficient. If it instead said 3130 and 2625, it would make no difference.
Because of this, turbine RPM is only reported in percent, which sometimes extends to the propeller on turboprops too. It is still aircraft-dependent, because some engines go up to values like 102.5% and the spread between maximum and most efficient also varies.
3. If we move prop lever we are changing blade pitch and then rpm adjust to it or we are changing engine rpm and blade angle adjust to new rpm
You are setting the desired RPM, which the governor will try to attain by adjusting the propeller pitch. If the propeller is too slow, the governor decreases the pitch, thus reducing the thrust and associated resistance until the engine can keep up, if it is too fast, it increases the pitch until the propeller absorbs all the power the engine is providing. If the throttle is too closed, even the finest possible pitch might have too much drag and the RPM will be lower than selected.
There were some variable pitch propellers where the pilot directly controlled the pitch, but the governor is a fairly simple hydromechanical device (weight on spring measures the centrifugal force and directly controls hydraulic valve that adjusts the pitch) and results in so much more practical handling that it quickly prevailed.
This is further complicated in beta range (reverse thrust) where the power lever switches to controlling pitch directly while throttle is controlled by another governor to maintain RPM. But you'll mostly encounter that on turboprops.

Thank you! Few more questions to your answer:

According to my first question, so the blade angle change is not sufficient to compensate all power loss due to lower rpm and therefore new net power output is lower right? And for example if I am flying with constant throttle setting and I decrease rpm, to maintain same power output I need to advance throttle a bit after reducing rpm?

On most Lycoming and Continental engines, the RPMs for the Tach are actually measured off of the engine. There is a cable running from the engine, through the firewall, to the panel. The cable connects to the back of the engine near the magnetos

And one more question: so power output is also regulated by rpm? For example if I fly at constant throttle setting and increase rpm despite having more fine pitch propeller I have a bit higher power input and inversely when I decrease rpm with the same throttle setting?

@Konrad, what do you mean by power "input" in the second half of your question?

It should have been output, my mistake.

@Konrad, don't forget the conservation of energy. Increasing the blade pitch does not increase power, it only increases the coefficient of lift, so the propeller will try to move more air with each revolution, but that requires more torque, and if you don't provide it by opening throttle, the propeller will just slow down to the available power (power = torque × rotation speed). And the engine can't develop as much power at lower rotation speed, so it decreases.

@DeanF., where it is measured is not relevant—it is a fixed transmission, so all parts of it turn at proportional speeds. What the pilot sees depends on how the instrument is labelled and that tends to be by propeller speed or percent.

@JanHudec - Granted it is not relevant where it is measured. It is only relevant to the other performance parameters of the particular aircraft. My comment was directed specifically  towards your comment that the Tach measures the RPMs of the propeller.

Horsepower is torque x RPM.  Reduce RPM, but maintain same torque (manifold pressure), HP goes down.  Reduce torque, maintain RPM, and HP goes down.  You'll find say, that 2300 RPM and 25" of MP make the same HP as 23" of MP and 2500 RPM.
If it's a direct drive engine, prop RPM and engine RPM are the same.  If it's a geared engine, prop RPM is prop RPM and engine RPM is engine RPM, and they are usually indicated with their own separate tachs.
When you move the prop lever you are adjusting the preload on a spring in the prop governor. The spring controls the force on a set of flyweights (like on a steam engine governor, but super compact).  The flyweights control oil to the prop to seek an RPM that that puts the flyweights in an equilibrium state (not moving out, not moving in).  The governor moves the blades to wherever it takes to get to that RPM with a given torque being applied by the engine.

I am really not sure of the wording of question #1. Taking a stab at it, I would say that reducing the RPMs at a constant throttle setting would increase the both the pitch and the AOA of the prop blades.
For question #2, it depends on the engine. Most certificated engines, the prop is connected to the flywheel/crankshaft in a direct drive configuration. So, the prop and engine move as one unit at the same RPMs.
For question #3, moving the prop lever adjusts the flow of engine oil being diverted to the prop hub. The pressure from this oil drives a piston which adjusts the prop pitch angle. The adjustment is done by controlling the speed governor attached to the engine. This creates a sort of feedback loop. I tried finding a picture of the L shaped weights of the governor and am unable to at this time.
See the following AviationStackExchange question,  How does a constant speed propeller work?

The only picture of a governor that I have. It is from a Lycoming IO-540.

I changed wording of question 1, maybe now it is clear. 2. But what if there  is a gear between prop and engine. In that case engine rpm are displayed?

In the only geared engine that I fly (the Rotax 912), the tachometer measures engine RPMs. The prop is connected to a reduction gearbox. I believe the Austroengine diesel in the Diamond brand of Aircraft also uses a reduction gearbox.


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What is the difference between SSR and ADS-B?
Can someone explain what is the difference between SSR transponder and ADS-B. From what I know ADS-B is a capability that can be incorporated into a transponder, but I do not understand the difference between them, I mean that from my point of view its' function and working principle is almost identical (both broadcast information that can be collected by ATC (and ADS-B info can be collected by other aircraft) and its like one device doing two identical things. Please help me out.

The biggest difference between SSR and ADS-B is how the aircraft position is determined. Secondary Surveillance Radar uses a radar antenna on the ground to send an interrogation signal and then determines the position based on the reply from the aircraft transponder:

The radar antenna rotates (usually at 5-12 rpm) and transmits a pulse which is received by the onboard equipment (transponder). The transponder sends back a reply containing at least a code (if operating in Mode A) but more often this is combined with level (mode C) or other information, e.g. aircraft identification, selected level, etc. (Mode S). The information received depends on the interrogation mode (A, C or S) and the transponder capability. [...]

When the reply is received, aircraft position (range and bearing) is determined. The range is calculated by knowing the time difference between the interrogation and the reply (the speed of propagation is the speed of light). The azimut is taken from the antenna position.

(SKYbrary - Secondary Surveillance Radar (SSR))
An aircraft equipped with ADS-B Out will regularly send signals without an interrogation. The aircraft position can therefore not be determined based on receiving such a signal. It must therefore be encoded in the signal itself, which requires that the aircraft knows its own position from a GNSS source:

ADS-B is a Surveillance technique that relies on aircraft or airport vehicles broadcasting their identity, position and other information derived from on board systems (GNSS etc.). This signal (ADS-B Out) can be captured for surveillance purposes on the ground (ADS-B Out) or on board other aircraft in order to facilitate airborne traffic situational awareness, spacing, separation and self-separation (ADS-B In).

(SKYbrary - Automatic Dependent Surveillance Broadcast (ADS-B))
ADS-B usually uses the same frequency of 1090 MHz to transmit, but it may also use other frequencies (see this answer for details).
Not only ADS-B signals can be received by other aircraft (ADS-B In). The ACAS (Airborne Collision Avoidance System) can also receive replies from SSR:

ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. ACAS II will not detect non-transponder-equipped aircraft and will not issue any resolution advice for traffic without altitude reporting transponder.

(SKYbrary - Airborne Collision Avoidance System (ACAS))


text:
Where can I find details about necessary system accuracy, integrity etc of a conventional navigation aid?
I am wondering where (probably in which ICAO document) can I find information about the required accuracy, integrity etc. for a conventional navigation aid for non-precision and precision approaches (required vertical and horizontal accuracy, etc.) I found that GNSS information can be find in ICAO Annex 10 Volume 1, but I coulnd't find similar clear information about conventional navigation aids.

These details can also be found in ICAO Annex 10 - Volume 1 Radio Navigation Aids. This is from the table of contents of the Seventh Edition (from 2018):

CHAPTER 1. Definitions
CHAPTER 2. General provisions for radio navigation aids
2.1 Standard radio navigation aids
2.2 Ground and flight testing
2.3 Provision of information on the operational status of radio navigation services
2.4 Power supply for radio navigation aids and communication systems
2.5 Human Factors considerations
CHAPTER 3. Specifications for radio navigation aids
3.1 Specification for ILS
3.2 Specification for precision approach radar system
3.3 Specification for VHF omnidirectional radio range (VOR)
3.4 Specification for non-directional radio beacon (NDB)
3.5 Specification for UHF distance measuring equipment (DME)
3.6 Specification for en-route VHF marker beacons (75 MHz)
3.7 Requirements for the Global Navigation Satellite System (GNSS)
3.8 [Reserved]
3.9 System characteristics of airborne ADF receiving systems
3.10 [Reserved]
3.11 Microwave landing system (MLS) characteristics


text:
Airliner - "barking dog" after take-off and before landing
I've always wondered what is that noise. It sounds like a barking dog or an air drill.
Sometimes you hear 4-5 "shots" but last time I flew the noise went on from before take-off to long after we were in the air.
It looks like they are tightening something. Am I correct?

This is the PTU, see reference in this question: http://aviation.stackexchange.com/q/137/572

Given that "barking dog" is the name everybody uses for this noise, Google would have found the answer for you straight away. Actually, I'm surprised the question we already have on the topic isn't high in the Google rankings.

I honestly expected to be told off for calling it "barking dog" :) So is it normal that the aircraft takes off with just one engine on? The answer made me a bit worried.

No, but it's not uncommon that aircraft taxi with one engine running. Takeoff and landing are are with both engines, hopefully.

What triggers the PTU (Power Transfer Unit)?

This sounds like the PTU, used to pressurize the hydraulic system when only one engine is running.
http://www.askthepilot.com/questionanswers/the-barking-dog-airbus-noises/

Somebody who doesn't know what the noise is probably isn't going to know what "PTU" stands for...

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text:
Why did this commercial flight fly in circles while far from the destination?
I was on the flight CX635 from Hong Kong to Singapore today, and I noticed the plane turning in a circle reminiscent of the holding pattern that planes often enter when they are queuing up to land.
However, this time I saw this flight pattern on the in-flight tracker while it was still in level flight, and once again when it was descending. When I got home, I saw that the pattern was also recorded on FlightRadar24.

Why did the pilot fly two circles in the middle of the ocean off the east coast of West Malaysia? I did not observe any obvious severe weather patterns, and the clouds were generally sparse (2 oktas or so) when it was doing the first roundabout, and there were no visible indicators of significant severe weather activity.

Running ahead of schedule for landing?

@SF. While that is plausible, the flight landed 8 minutes late, and furthermore from my experience, they usually do the circling near the airport if they are unable to be cleared to land immediately, and this is the first time I have seen planes circle while nowhere near the airport.

@MarchHo "near" is relative when speaking of aircraft travelling at hundreds of knots.

Having holding patterns that far out from the airport are not uncommon. It's not usually so much a matter of the aircraft being 'early' relative to its published schedule, but rather the destination airport being busy and them not being sequenced for landing as early as they'd hoped. Since the plane can't just stop to wait for its turn in the sequence, it has to fly a holding pattern. Also, like Federico mentioned, 'far' is a relative term here. 150 miles (approximately the distance here) is only about 15-20 minutes at the speed of an airliner.

Wow, quasi-perfect holding pattern!

If something unexpected happens like a burst tire on landing, ATC has an instant "airshow" to handle close to the airfield, until truck has gone out and cleaned up the debris. If they can "press the pause button" on incoming flights well before they reach the logjam around the airport, based on an estimate of how long it will take to clear, they will do exactly that. Out over the ocean a high altitude, you aren't annoying anybody by making noise and you are burning less fuel (lower air density = less drag force) than if you were holding at a lower altitude.

The first "circle" is a holding pattern. We discussed those here.
In particular, it seems that you were on hold at VINIL or VEPLI, according to the approach charts (page 5 of the PDF).
The second one is most likely for separation on final approach, the aircraft was probably early and ATC has asked them to perform a 360° to increase the separation between them and the aircraft in front.

Great answer. It's interesting how inefficient the system is, in regards to fuel consumption. The first circle is not that long after takeoff. I wonder if the destination airport knew about the congestion before the flight even departed. Maybe the originating airport wanted the space, or perhaps the airline didn't want a history of late flight departures, so they left on time, but then flew around in circles as directed by ATC. They obviously wasted a fair amount of fuel.  As a side note: The map does not reveal if they performed the circle/oval patterns once or more than once.

@RockPaperLizard, those circles are as the plane was coming in to the destination (Singapore), not leaving the originating airport (Hong Kong).

@JPhi1618 Thanks!  I was holding the map upside down.  ;-)

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Why does engine temperature rise upon applying carburetor heat?
When carb heat is applied, the mixture enriches due to hotter and less-dense air mixing with the fuel. But a richer mixture has the side effect of cooling the engine down. So why does carb heat cause the engine temp to rise?

You need to be specific about which engine temperature you mean, oil temperature, cylinder head or exhaust gas temperature. Is this a theoretical question or are you actually seeing this on an airplane?

You mean cylinder head temperature?

@gdd It's a theoretical question that I got after reading that carb heat reduces the RPM and increases the engine temp. I'm not quite sure which temperature is being referred to, but am now curious how it affects each one of them. Thanks!

Carb heat is recycling exhaust heat back through the cylinders, so all temps will rise some small amount, eventually.  EGT right away, then CHT after maybe a minute or two, then oil temp.

Switching Carb Heat to ON sends air to the carburetor that was heated by being near the exhaust pipe.  While it is correct the fuel/air mixture will be richer, the amount of fuel entering the engine does not increase.  A slight drop in RPMs may have a some cooling tendency but the heating of the incoming air off-sets this.  Gasoline absorbs much less energy during evaporation than water$^1$.
Richer mixtures do burn with a lower temperature, but again the heating of incoming air results in a net gain of engine temperature at the cylinder head.
$^1$ water/methanol injection can be used help control engine temperature, but may be corrosive to engine parts

Loss of power with Carb Heat ON may actually cause the pilot to increase throttle.  Also, as the exhaust gets hotter, cylinder head and oil temp increases will follow.


text:
How close does a plane have to come to the ground before triggering the ground collision alert?
The recent AF953 near-miss incident as reported by the BBC describes the ground collision alert warning triggering, preventing a CFIT by the plane.

An initial incident report published by BEA said an automated warning,
  saying "Pull up", was activated. The plane then ascended quickly from
  9,000ft (2,743m) to 13,000ft (3,962m) to avoid crashing.
BEA did not say how close the plane came to Mount Cameroon.

How close does a plane have to come within the ground (as measured by the radar) before the warning triggers? Does this number vary significantly between aircraft models, or is there some legislated fixed value for the distance?

AFAIR it is fixed time.

I found an graph on wikipedia showing the altitude triggering an alarm depend on the sink rate

This diagram comes from page 37 of TSO-151C and only shows Mode 1. There is not a simple, one size fits all answer to this question.

There are 7 modes of warning
Mode 1. Large descent rate near terrain. Gear and flap position not a factor in the warning.
Mode 2. Large closure rate with terrain.
Mode 2A. Occurs for a large closure rate if the flaps are not in the
landing configuration and glideslope deviation is more than 2
dots.
Mode 2B. Alerts for a large closure rate if the flaps are in
the landing configuration.
Mode 3. Alerts for a large altitude loss during takeoff or
during a missed approach with the flaps not in the landing
configuration or landing gear not down.
Mode 4. Alerts when the airplane is too close to the
terrain and the landing gear or flaps are not in the landing
configuration
Mode 4A alert when the landing gear is not
down and gives the aural messages TOO LOW GEAR at
low airspeeds, or TOO LOW TERRAIN at high airspeeds.
Mode 4B alerts when the landing gear is down and the flaps are not in landing configuration and gives the aural messages TOO LOW FLAPS if the airplane is at low airspeed, or TOO LOW TERRAIN at high airspeeds.
Mode 5 alerts when the airplane goes below
the glideslope during approach if the landing gear is down.
Mode 6 gives aural callouts when the airplane descends through set altitudes with the landing gear down. It gives callouts like MINIMUMS or DECISION HEIGHT.
Mode 7 gives windshear warnings.
Every one of these modes is altitude, speed and aircraft configuration dependent.
In the Enhanced GPWS, these triggers are also changed based on the position of the aircraft.
Regulatory requirement 14 CFR 121.354
Technical requirements TSO-151C

Do you have a source for your answer? Also, your answer does not answer the question at the moment.

The source is Boeing but it is common knowledge. It answers part of the question which is 'How close does a plane have to come within the ground (as measured by the radar) before the warning triggers?'. The answer to which is really, it depends on the situation.

As for regulatory requirements, 14 CFR 121.354 is a starting point. FAA TSO-151C are the system requirements.

@SportsRacer I think if it were common knowledge the question would not have exist. And you really should provide links and references to external resources to enhance your answer

@ManuH its common knowledge if you operate an airplane with EGPWS.

@casey A significant part of this site's users asking questions (including me) does not have more than few tenth of flight hour. Also consider the part of users whose knowledge is limited to general aviation. See a random subset of user's profile to have a better idea of how common a GPWS could be used among this site's users.

Am I correct to say that in the case of the event reported, Mode 4 would have triggered (with the "Terrain" warning)?

I think 4A would be correct.

The GPWS doesn't use radar but rather a terrain database and a GPS position. Alert are given based on time before collision rather than a fixed distance, since a distance that provides inadequate reaction time for a fast aircraft might be unreasonably large for a slow one, and it is important with warning systems like this that all alerts be valid ones, not false alarms that one gets in the habit of ignoring.
Times can vary by system, but I think it's generally about 1 minute away you get a "caution" message, and 30 seconds away, a "pull up" warning.
Link to Wikipedia article -- gives some GPWS theory but doesn't address algorithms: http://en.m.wikipedia.org/wiki/GPWS

Enhanced GPWS (EGPWS) uses a terrain database and GPS, however the most basic system and even EGPWS use the aircraft's radio altimeter (often called a Radar Altimeter) to do it's calculations.

This is a better link for information. http://www.boeing-727.com/Data/systems/infogpws.html

It depends on the type of aircraft and the avionics systems onboard.  A STOL aircraft may not give you any warnings at all.  Garmin G1000 systems are entirely configurable to the pilots settings.  I personally have mine set to alert me when I'm at 500ft from any terrain.


text:
Why do in-flight entertainment systems separate the left and right channels in their earphone jacks?
I have seen a number of in flight entertainment systems which utilise two 3.5mm jacks for their earphone jacks, instead of the 1 jack with 2 channels normally used in almost all applications.
An example (from Singapore Airlines) is shown below.

A normal 3.5mm 2-channel jack is shown below as contrast:

Why is this done only in airplanes (to the best of my knowledge)? Is there a reason behind this separation of audio channels?


text:
Do aircraft cabins have suspension?
On cars, we have 'shock absorber' suspension that decouples the passengers and loads from the road vibrations, bumps etc.
On planes do we have any form of suspension that decouples the passengers & loads from the turbulence, and vibrations induced by 'bumps' in the air?
I imagine even if it doesn't use a spring damper system, whatever attaches the cabin to the outer shell of the plane will have some finite stiffness. If so, has damping of this spring been considered to avoid oscillations?

I think it should be on the chassis if the wheel can be lifted up to replace the tire, but I understand the question is about something that is active also in the air.

Springs? Even better - all aircraft are riding on air suspension, which, as any automotive writer will tell you, is the smoothest ride you can get!

Besides the cushioning in your seat?  Not that I have ever seen.

I know you asked about turbulence and vibrations in the air, but the landing gear on an commercial aircraft have shock absorbers like cars. They are like struts on cars, built into the gear and use hydraulic dampers.

The cabin is rigidly attached to the fuselage, with bolts. But in a way the cabins do have suspension: from the wings riding on air.

Elasticity of the wing construction material acts like a spring. Load it with a vertical gust, and it will bend upwards, then spring back.
The wing bending experiences damping forces from the surrounding air, proportional to bending velocity.
The wing sweep angle spreads out the gust: not all of the wing is accelerated upwards immediately.

Modern subsonic airliners with swept wings, flying with near transsonic speeds, experience much less turbulence than the olden days DC-3 type aircraft. So far, so good.
OPs proposal to provide suspension between fuselage and cabin may not be very effective. Notice that car suspensions work best when the unsprung mass is low: independent suspension is able to follow the un-evenness in the road much more effectively than rigid axle suspension, due to the much lower unsprung mass of only the wheel+tyre.
The unsprung mass of the suspended-cabin would be the whole fuselage-plus-wing structure, engines, fuel etc. Considerably more than that of the cabin plus payload. A lot of extra complexity for not much gain.

The comparison between modern pressurized aircraft and DC-3 is unfair (but yet valid): the DC-3 flew at a lower flight level, where air is more turbulent.

Weren't wings built much stiffer in the days of the DC-3, too? That would make for a rougher ride.

@aCVn Lower wing loading makes for a stiffer ride indeed.

You had "uspended- abin". I changed that to "suspended-cabin" since it looked like you'd just lost the first letter of each word. I was thinking, though, that you meant "unsuspended-cabin", but the more I read it, the less sure I am. I'll leave it to you to edit again if necessary.

@FreeMan Thanx.

Technically, yes, but not in the way you envision, and only in academic quantities.
Any mechanical system has some damping in it due to hysteresis in the materials, friction on the joints, and elastic bending of the structural members.
An aircraft's wings bend, providing the spring component of a shock absorber, and dissipate energy via hysteresis and friction, providing the damper (dashpot) component. The overall effect is much smaller than that of a dedicated shock absorber, but it exists.
Do note that all the above effects are generally undesirable in an aircraft and are not there by design. Elastic bending in particular can couple with the aerodynamic forces and lead to flutter, control reversal and structural divergence, for example.

“not there by design” – really? I would have thought the wing elasticity is quite important to avoid load concentration from cracking the lightweight structure.

@leftaroundabout the fact that elasticity is not wholly inconvenient does not mean it is designed in; in fact I am yet to see a design or strength analysis document that specifies a designed elasticity minimum. Stress concentration in the wing structure is avoided by careful geometric design and large safety factors, not by deliberately choosing more elastic materials to absorb energy.

Well, this might merit asking a new question about.

@leftaroundabout Aircraft wings are pretty flexible. See https://www.popularmechanics.com/flight/g2428/7-airplane-wing-stress-tests/ for example. But flexibility on its own doesn't provide any damping at all.

@leftaroundabout Flexibility doesn't necessarily reduce load concentrations either, unless there are several different load paths and the total load can be redistributed between them. For a statically determinate structure like an aircraft wing (which is basically a cantilever beam attached to the fuselage) that does not apply.

@alephzero flexibility doesn't provide damping, but it does increase the damping ratio achieved by the same amount of damping by other mechanisms, so the dynamic response to a given turbulence input still benefits from the flexibility as far as comfort is concerned.

Let us continue this discussion in chat.

I flew in an ATR-42 with a computer in the cargo bay.
I wish the propellers had more shock absorption, because vibrations literrally ejected and broke the DIMM RAM Clips from their slot. As a positive outcome, sugar was mixed in my coffee in no time with the vibrations.

No, they do not. Aircraft rely on the properties of the materials they are built from to absorb such forces. Adding springs would merely increase dead weight and lower profitability, while providing little added relief from temporary bumps. Besides, anyone who has ridden in anything with shock absorbers knows that the bumps are not eliminated completely.

Correct me if I'm wrong, but aircraft turbulence normally has a much longer period than road potholes.  So it's more like going over a giant speedbump, and the cases where you wish you might wish you had "suspension" would require that suspension to travel most of the height of the cabin to isolate it from the movement of the wings.

@PeterCordes Longer period, yes, but an aircraft travels much faster than a car, which has the effect of reducing that period.

Are you contradicting yourself there, or are you just saying that it is longer than in cars but would be even longer if planes didn't fly so fast?  Or did you mean to talk about longer "potholes" (in the direction of travel) but higher velocity resulting in still a fairly short period?  What really matters of course is the total vertical amplitude of the bumps relative to the height of the airplane fuselage, not wavelength: if it's going up or down by a large fraction of its own height, there's not much you can do to damp the acceleration felt inside the cabin.

@PeterCordes You were the one who mentioned period in the first place, not me. If you want to talk amplitude now, fine, but I'm not the one contradicting myself. :)

I just wanted to clarify whether you meant that "longer period than a car" was actually still true, or if you meant that fast-flying planes shorten the period so much that it is like a car after all.  That sentence seems to be saying two opposing things.  Anyway, high amplitude low frequency vs. high amplitude high frequency is an important difference which is why I was bringing up period.  To damp low frequency oscillations, you need really weak coupling which is maybe harder to do without often hitting suspension-travel limits hard and getting extra jolts.

The cabin and outer shell are one structure. The structure is built up from ribs (circular pieces) and stringers (lengthwise pieces) which form a sort of framework. An outer skin is bonded (adhesives and/or rivets) to this framework; the whole things becomes one integrated structure. The floors are attached to (and contribute to) the whole structure. Interior finishing (wall/ceiling) panels are rigidly (more-or-less) attached to the inner surfaces of the ribs and stringers - the space in between is filled with insulation. There is no mechanical isolation between the exterior and interior. Any suspension effects come from the properties of the structure itself - a certain amount of flex - and the passenger seat cushions.

It is important for handling that the cabin is not decoupled from the rest of the plane.
Suppose you have your suspension system. The plane hits a thermal and there's a bump. Now anyone who's pushed on a corner of a car knows it doesn't just stop - it overshoots slightly and comes back again. On your hypothetical plane, the pilot will get one jolt from the thermal, but then another jolt the other way as the suspension overshoots and comes back, and then another small one coming back again, typically. Controlling this would be nigh on impossible.
Henry Bessemer had a similar idea for ships. The SS Bessemer was the result. She was a complete failure - she only sailed once, and destroyed the mooring pier at Calais because she was impossible to steer.

Overshoots are controlled by choosing appropriate damping. Overdamped systems never overshoot.

The SS Bessemer is an interesting case though, +1. Nowadays active suspension would be possible, like a motion system underneath a simulator. I have in fact seen 6-DoF motion systems that are used as stabilising platforms on board of ships.

@Koyovis But overdamped systems don't soak up the bumps very well. And even active suspension will tend to do something similar - model-predictive control is pretty good, but inertia and overshoots still happen. A stabilised platform works because it isn't a significant fraction of the overall mass of the ship; the SS Bessemer shows what happens with most of your mass running on gimbals though. It might be possible to use fly-by-wire to run it, I agree, but it's a complex control problem, and generally that's not a safe place for an airliner to be.


text:
Why are so many ATR involved in accidents in the past years`?
I asked myself a question why are so many ATR involved in accidents in the past years?

ATR-72, 16 October 2013, Lao Airlines Flight 301
ATR-42, 19 October 2013, P2-PXY, Air Niugini
ATR-72, 23 July 2014, TransAsia Airways Flight 222
ATR-72, 02 February 2015, TransAsia Airways Flight 235
ATR-42, 16 August 2015, Trigana Air Service Flight 257

I can't see any useful comparison to be made without a lot more information.  What is the list you provided as a percentage of the overall list compared to number in service as a percentage? How many flights? Where do they operate? What kind of flight profile compared to other aircraft/operators/locations? What are the relative statistics for the airlines? Do they point to training/maintenance issues?  etc etc etc.

Remember you are speaking of a rare events, and thus it is difficult  to make statistics based on those events. You may compare accident list with other aircraft whose number built is less than 1000

ATR-72, 16 October 2013, Lao Airlines Flight 301

The probable cause of this accident were the sudden change of weather
  condition and the flight crew's failure to properly execute the
  published instrument approach, including the published missed approach
  procedure, which resulted in the aircraft impacting the terrain.

ATR-42, 19 October 2013, P2-PXY, Air Niugini
According to released information this cargo aircraft overran the runway after the crew aborted the take-off above Vr after experiencing pitch control problems. The final report of this accident is not yet published but a preliminary report is available.  The investigation focusses on the payload distribution which may have caused imbalance leading to the perceived control problems.
ATR-72, 23 July 2014, TransAsia Airways Flight 222
The aircraft crashed on final approach in low visibility conditions (800 meters). During the initial approach the crew had requested and ILS approach (precision approach) to runway 02 but then changed their minds and asked for a VOR approach (non-precision approach) to runway 20. During the approach, after deactivation of the autopilot, the aircraft deviated 340 meters to the left and the crew subsequently announced they were going around. Shortly afterwards the aircraft hit tree tops and impacted residential buildings.
The final report is expected in October 2015.
To me it seems the probable cause is the failure of the flight crew to properly execute the published instrument approach including the published missed approach procedure which resulted in the aircraft impacting the trees and buildings.
ATR-72, 02 February 2015, TransAsia Airways Flight 235
The investigation into this accident is still running, only some preliminary information has been published. Preliminary data indicates that about 36 seconds after takeoff the engine no.2 auto-feathered, about 46 seconds subsequent to auto-feather the engine no.1 was shut off by the crew, resulting in loss of all thrust. The aircraft subsequently descended, stalled and crashed into a river.
ATR-42, 16 August 2015, Trigana Air Service Flight 257
This accident occurred just a couple of days ago so there are few facts known yet and no preliminary findings are published. I will not speculate about the probable cause of this accident.

The common thread in the first four accidents seems to be the failure to comply with established procedures resulting in the loss of the aircraft.
In two cases (Lao Airlines Flight 301 and TransAsia Airways Flight 222) the flight crew flew a controllable aircraft into the ground.
In the case of the Air Niugini crash improper loading of the aircraft is likely to be the cause, resulting in the aircraft being not trimmed correctly for take-off.
The crash of TransAsia Airways Flight 235 is the only crash that involved an aircraft system malfunction (engine 2 failure). However, the ATR-72 can fly on a single engine so the failure itself should not have been fatal. The crew then shut down the other engine (number 1) which sealed the aircraft's fate. It is too early to conclude that the crew didn't follow the correct procedures here; for example there may have been ambiguous indications from the engine instruments that led them to believe that engine no. 1 had failed. However with the information currently available it seems that the crew did not follow the correct procedures for shutting down a failed engine.
The high number of crashes involving an ATR aircraft seems not to be related to the model itself, but to the safety culture and lack of training within a number of airlines operating the aircraft.

More of an issue of pilot training, weather, safety of the airline and the country's regulator in general. ATRs are popular in regions that encompass many islands, hence the application in Southeast Asia. Some of these airlines are "poor" so they cannot afford to operate larger jet aircraft, nor would operating such an aircraft make sense in economical terms for many of the routes they operate. Would say that it is circumstantial and not a problem with ATR aircraft, per se.

Blaming flight crews is a standard procedure by almost all manufacturers.
Steve Frederick, an ATR 72 pilot was one of the many that noticed loss of flight control problems in icing conditions and warned American Eagle but was ignored. A friend flying ATR 72's commented "I'll probably die on one of these planes", and did on American Eagle Flight 4184. The company response was to fire pilots commenting on icing common on these planes. Many countries and companies grounded these planes, some banned flight in colder areas. I was aware of the problem and once noticed ice on the wings of one and avoided flying in these models. The chord-wise extent of the wing deicing boots on the ATR-72, while sufficient for milder conditions, was determined by the investigators after many incidents, was proved to be inadequate to protect against the large droplet icing conditions and resulted in many accidents and deaths.

This answer should (at most) be a comment. None of the accidents listed in the question have a cause related to icing. I am aware of two other accidents that involved icing after flight 4184. Aero Caribbean  Flight 883 and TransAsia Airways flight 791 involve large droplet icing, but I don't know of "many accidents" related to large droplet icing with the ATR 72. Do you have any references?


text:
Has the F-35 Lightning II overheating problem been resolved?
This is one of the sources explaining the F-35 overheating problem. Has this been resolved?
Are the constant problems, that are being reported about this type normal for an aircraft like this?

I like how the main image on the page isn't an F-35.

Yes, that certainly is journalistic achievment par excellence, the pulitzer must be right around the corner.

This is the Daily Express we're talking about. The only thing remarkable about that story is that it doesn't blame immigrants or have something to do with the death of Princess Diana.

It hasn't been solved. https://mainichi.jp/english/articles/20190417/p2a/00m/0na/006000c

As to the first question, it appears as fuel cooling of electronic components and other equipment is integral in the F-35 design, all branches of the armed services will not be able to remedy this problem and it will have to be solved with only fueling the aircraft with cold fuel.  The Air Force has opted to use fuel trucks painted white in order to keep fuel cool while on the ground and it may require refrigeration equipment in fuel tank farms at F-35 bases at an additional cost to the program.
The fuel problems for the F-35 are but one of several hundred faults, shortcomings and outright vulnerabilities with the troubled Joint Strike Fighter program.  While developmental issues are common in new aircraft programs, none so far have been this serious, so numerous and so overbudget without being killed.

Are there any comparisons of such programs? From where do you have the information, that other programs like this were killed?

The A-12 Avenger, the RAH-66 Comanche, and nearly the ATF/F-22 quickly come to mind.

As a yardstick for comparison, the F-14 airplane went from concept to new F-14As rolling off the production lines in Bethpage and entering the fleet in 51 months.


text:
Are any airlines actually flying the Comac ARJ21?
The Comac ARJ21, China's answer to Embraer's regional jets, was launched with great fanfare in 2016.  However, if FlightRadar24 is to be believed, launch customer Chengdu Airlines does not appear to be flying any of its aircraft (click open AJ27, then check the aircraft).
Are any airlines actually flying the Comac ARJ21?  If not, when and why did they stop?

You are correct in that the FlightRadar24 site doesn't show any flights for the ARJ by aircraft. But going to the routes tab and searching through the routes there are ARJ assigned to some routes. So it appears that Chengdu Airlines is operating the aircraft at least on a limited basis.
I would also say that while FlightRadar24 data is generally very good, it isn't perfect. They do rely on data from external sources and considering the Chinese government's information and internet policies, it's entirely possible that the data availability is not the most reliable.

FR24 shows lots of flights for other Chengdu Airlines aircraft though.  Perhaps the routes are just based on where they used to fly the ARJ21?

Don't know.  There's not a lot of transparency when it comes to China.  But one thing I can say without reservation; airlines don't like airplanes they own to sit on the ground. They're expensive assets and they need to fly to generate revenue.

That's why I posted this, because it looks like there's something badly wrong with the aircraft if nobody is flying it...!

They may be flying.  FlightRadar24 gets its data from ADS-B. China is only requiring airlines to be 50% equipped by 2020.  Full equipage is not until 2025. It's possible the ARJs are not equipped and that's why they're not showing up on FlightRadar24. I read elsewhere COMAC is planning a major cockpit retrofit for the ARJ21. It may take that to get the full compliance for ADS-B.

Interesting!  Poked around a bit more, and I found one apparently ARJ-operated flight last week, but that's based on the schedule and there's no ADS-B data: https://www.flightradar24.com/data/aircraft/b-3329

I noticed an ARJ21 on FR24 last week, so Chengdu Airlines are definitely flying some of their ARJ21s now: https://www.flightradar24.com/data/aircraft/b-602c

Ghengis Kahn airlines is supposed to start flights with the ARJ21 this spring, and received their first ARJ21 airplane last month. But I'm not able to find any flights with the plane yet.
https://www.flightglobal.com/news/articles/pictures-first-arj21-delivered-to-genghis-khan-air-456044/


text:
Can ICAO airline designators be reassigned to a new airline?
Is it possible for three-letter ICAO airline designators to get reassigned?
In particular, I'm trying to find out if BKL was previously used for Baikal Airlines (ceased to exist in 2001) and reassigned to Barkol Airlines (founded 1996, although it's unclear when they received their ICAO code).

Yes. As per rules ICAO can reassign the three letter airline designators after a period of 60 days.

When a designator is no longer required, ICAO should be informed immediately. Any such designator so released will not be reassigned until a period of at least 60 days has elapsed.

The designator BKL was used by Baikal Airlines and is currently used by Aircompany Barcol.
Another example is the airline designator WOW, which was used by Air Southwest  and is currently used by WOW air.

Thank you!  Are there historical records of who owned which code available anywhere?

I found this site quite useful.


text:
What is the device at the tail of the F-16 Block 60?
The F-16 Block 60 has a lot of advancements, one of which is the device at the tail. What is it?

For clarification, I believe all these vents (second image) are for said device.

Is this an Israeli F-16? The only F-16's that I've seen with conformal fuel tanks are Israeli ones, and all the mods on this one make me think that the tail feature is some special Israeli modification of the F-16.

It is a Block 60, that makes it Saudi. AFAIK CFTs are an option for a lot of versions of the F-16.

Those conformal tanks ruin the lines of one of the prettiest fighters.  Oh well, it's better to not run out of fuel than it is to be pretty.

That’s a ram air intake and exhaust for a heat exchanger used to cool the Falcon Edge Integrated Electronics Warfare Suite (IEWS). The system remains fairly clandestine, but a few of its components are known to exist.
https://www.northropgrumman.com/Capabilities/FalconEdgeIEWS/Pages/default.aspx

The linked article references the IEWS, but nothing about the heat exchanger. I looked at other articles, no mention of the device.

I've come to the same conclusion as @Carlo Felicione, but I agree with mike, there needs to some evidence this is a heat exchanger. It most propably is, but not for sure. No upvote at this point.

How does the IEWS look like exactly? I'm not sure that plane has it installed.

@mike it’s an internal avionics package, so it’s going to be a series of LRUs fitted into an avionics bay on the jet or various places.  We do know the system uses an active digital jammer and these often consume a lot of power and get quite hot during operation, hence the need for an heat exchanger.


text:
Can a passenger drone perform auto-rotation?
(Source)
The Dubai Drone is an Ehang passenger drone planned to start commercial services in July 2017.
If there is a failure, can such an electric multi-copter auto-rotate a landing?
I'm wondering, if an electric motor would even allow auto-rotation, and if the autopilot could handle it? Are there regulations for devices like that?

It looks like you could attach a parachute to that, at least.

It is presumably possible to build a "drone" rotorcraft with an autopilot capable of flying an auto rotation of a suitable rotor system (though the make the best of a bad situation logic in choosing a forced landing site in unsuitable terrain may be challenging) however the rotor systems of that particular aircraft is not suitable.  You'd use independent power systems and/or a ballistic parachute as JAB mentioned.

Why are people referring to these aircraft as "drones"? A drone, by definition, is robotic and lacking intelligence for rational self-interest (so are generally autonomous for our benefit or are remotely piloted) - it seems people are now generally referring to all quadcopter or multi-rotor aircraft as "drones".

This device is a drone, because it is remotely piloted.

That drone has fixed-pitch rotors, and that pitch is optimized for thrust, not for autorotation. In the absence of power, those rotors won't autorotate. They will stop rotating 'in the right way' and then start windmilling in the opposite sense. That windmilling will cause drag and some deceleration of the fall, but not of the same magnitude as an autorotation, that is a different condition.

So it will drop quickly?

@mike And out of control. The only control of the rotors in that type of drone is by modulating the power of the motors, since the rotors themselves are fixed-pitch and not fully steerable. In the absence of power, there will be little control, if any...

Also, the inertia of the small rotors looks too small to store enough energy for deceleration. Even if id did have variable pitch props, autorotation will only reduce the sink speed a little.

@Peter Kämpf  I'm not sure... The total disk area is relatively large... It's true that the energy of the rotors is very small, making a final landing flare impossible, but the sink speed will be considerably reduced... (in true autorotation, achieved with the right pitch setting...)

Are there any regulations that guide the development of drones like that? What kind of failure would result in a situation, where autorotation would save a passenger?

@mike if one motor were to fail, then it could probably execute a powered landing quite safely, though I wouldn't want to do any more than that - it would likely be unstable and underpowered.  In this way, the passenger would be saved, but it's not autorotation.

@Baldrickk I know, that's why I'm asking about autorotation, because my guess is, that there are quite a few scenarios, that will lead to loss of all rotors, which turns this into the more interesting case. While multicopters are hailed as safer, it turns out they aren't, because they don't autorotate. Imagine living in a city, where several 100 of these things fly and could just drop out of the sky.

The way you would make something like this safe would be to have sufficiently independent power systems for each motor that they wouldn't all go down at once.

Simple answer. No.
A drone with fixed wing propellers such as the one you describe can not autorotate. For autorotation to work the pitch of the blades needs to be variable.

At the instant of engine failure, the main rotor blades are producing
lift and thrust from their angle of attack and velocity. By
immediately lowering collective pitch, which must be done in case of
an engine failure, the pilot reduces lift and drag and the helicopter
begins an immediate descent, producing an upward flow of air through
the rotor system. This upward flow of air through the rotor provides
sufficient thrust to maintain rotor rotational speed throughout the
descent. Since the tail rotor is driven by the main rotor transmission
during autorotation, heading control is maintained as in normal
flight. - https://en.wikipedia.org/wiki/Autorotation

A drone or multicopter with 6 or more propellers can continue flying as long as the thrust provided by the remaining propellers and engines is sufficient to keep it from falling.
The aicraft may however begin to rotate around its own axis. In normal flight there are an equal number of clock wise and counter clockwise rotating propellers. If a propeller/motor stops spinning the torque of these engines will no longer cancel each other out so in order to not descend the helicopter may be forced to accept a rotation around the Z axis. This same mechanism is used for turning a multirotor, lower the RPM of the propellers spinning in one direction and you will turn in the other.
I fly RC drones myself and have tested how my hexacopter behaves during a motor failure. The conclusion I came to is that if it was close to its carrying capacity it would rotate since it needs all the power it has and can't worry about rotation. But if it only carried a light load it woulden't rotate as it has power left over to compensate for it.
There are however a few attempts at making drones with variable pitched propellers
This however negates the pure simplicity that a drone provides as one of it's main advantages is that there are only as many moving parts as there are propellers.
In my opinion and others (http://diydrones.com/forum/topics/why-are-we-not-seeing-more-variable-pitch-quadcopters) the only way to provide a reliable drone is to have 6 or more propellers and lots of power to spare.

"A drone or multicopter with 6 or more propellers" You mean 4. One could design a drone that had 4 rotors that could handle a motor out condition.

@Sam true, although I have yet to see one. Also, with only 4 props loosing one would mean loosing a substantial amount of the total thrust which would mean that you would have to create something very overpowered or be forced into a descent upon failure. In addition the CG of the craft would be much more crucial and even a small mistake or misloaded cargo would cause a crash.

@qqjkztd What? "heptarotors are safer than octorotors" they may be more "safety efficient" but are not generally safer. JensB is correct the torque of rotation is a design problem with odd rotor numbers. Also when you get beyond 1 lost rotor, you have to account for where and how the symmetry works out.

@qqjkztd then you cant turn as the multicopter uses the torque of the engines to turn.

Another intresting point in this is that the multicopter falls like a rock if any of the electric systems fail (which in my hobby experience is waaayyy more common than broken engines/props) where as a normal helicopter can keep going or at least land in most cases. Personally I would rather travel in a regular helicopter any day of the week.

@Sam, how do you handle engine out with just 4 engines? You have 3 left, but because the CoG is between the two, the third mustn't be producing any lift, which it can't do without variable-pitch propeller. And if you cut it, you lose roll control on that axis.

@qqjkztd, I understand it has 8 engines, each driving its own propeller. If it was just 4 engines, it wouldn't be controllable with one engine out.

@JanHudec , I said one could be designed that way. It wouldn't work in the configuration you are used to. You could design one with the engines in a pyramid, for example, or with 3 arms and a center engine.

@Sam, centre engine won't help unless it alone can keep the thing flying. Pyramid would, since you can tilt to move the CoG inward, but it would need tilting engines.

@JanHudec I didn't say it wasn't a  trivial case, I just said it could be done.

@JensB have you signed up to the Drones proposal on Area51? https://area51.stackexchange.com/proposals/107036/drones

I am a career helicopter pilot who enjoys watching the development of drone technology.  I can tell you for certain that this aircraft is incapable of autorotation.  A parachute would be it's best option for pure simplicity.  This would not guarantee a safe landing, just avoid a 'splat'.  It is very apparent to me that the number one challenge of the drone industry as it relates to increased payload (passengers, cargo, luggage) is power.  Drone advancement is highly reliant on battery advancement.  If you have a lightweight, high powered battery system, then the sky's the limit for drones, safety and otherwise.  Short of that, I'm still waiting for the big breakthrough. For now, I'd just like to see y'all drone drivers get an anti-collision light.  Oh wait...more power is going to be needed for that :)